The present invention concerns a high voltage generating circuit for an electrically erasable and programmable semiconductor memory device (hereinafter referred to as "EEPROM"), particularly a circuit for readily adjusting level of the high voltage used in a flash-type EEPROM.
Generally, an EEPROM employs as a memory cell a floating-gate field effect transistor with an insulated floating-gate between a channel and a control gate to which a gate voltage is applied, so as to erase or program data by utilizing the electron tunnel effect between the floating-gate and the active region of the transistor.
In erasing the data, a high voltage of 15 V to 20 V is applied to the gate electrode for the electrons to tunnel from the source or drain region of the transistor to the floating-gate. Additionally, there is usually adopted a method to erase the data of all the memory cells at once in the case of a flashtype EEPROM. In programming, the floating-gate field effect transistor serving as a memory cell is made into a depletion type by grounding a control gate electrode thereof and applying a high voltage to a drain thereof. Hence, the EEPROM generally includes a high voltage generating circuit required for carrying out the above erasing and programming operation.
Referring to FIG. 1 for illustrating a conventional high voltage generating circuit, there are connected in series a plurality of pump circuits 1 which responds to pump clock pulses 4, 4' produced from an oscillator. A pump circuit 1 of a first stage is connected with a NMOS transistor 2 whose drain and gate are commonly connected to a source voltage terminal Vcc. The pump clock pulses 4, 4' have opposite phases.
The output of the pump circuit of a final stage is connected with a high voltage output terminal 10 that is to apply an erasing voltage of high level to the control gate electrode of a memory cell. The erasing voltage is provided through a row decoder to a corresponding word line or through a programming latch circuit (not shown) to a bit line that are connected with the drain of the memory cell.
Between the high voltage output terminal 10 and the source voltage terminal is interposed a N-type pull-up transistor 5 with the drain and gate connected to the source voltage terminal. Also, between the high voltage output terminal 10 and a ground voltage terminal is interposed a N-type clamping transistor 3 with the source and gate connected with the ground voltage terminal.
Each respective pump circuit 1 comprises a pair of capacitors 6, 8 having one electrodes connected to the pump clock pulses 4, 4' and the other electrodes connected to the gates and drains of NMOS transistors 7, 9. The NMOS transistor 2 drops the source voltage Vcc by a threshold voltage Vth thereof. The dropped voltage is applied to the drain and gate of the NMOS transistor 7 of the pump circuit 1 that responds to the pump clock pulses 4, 4' so as to increase the dropped voltage to a given high level. It will be readily appreciated by one having the ordinary knowledge in this art that the level of the high voltage output through the high voltage output terminal 10 depends on the number of the pump circuits 1.
Meanwhile, the pull-up transistor 5 serves to maintain the voltage of the high voltage output terminal 10 at Vcc-Vth when the pump circuits 1 are not operable. Though the clamping transistor 3 is a NMOS transistor, it operates in a breakdown region because the gate thereof is connected to the ground voltage terminal.
In such a conventional high voltage generating circuit, whenever the pump clock pulses 4, 4' are applied to the pump circuits 1, the voltage output of the pump circuits 1 is increased, and the increased voltage is clamped by the clamping transistor 3 in the breakdown level thereof.
Consequently, the clamping transistor 3 directly receives the stress caused by the high voltage output, and therefore it is liable to be destroyed. Moreover, the clamping transistor 3 is affected by the variable factors involved in the manufacturing process thereof, so that the breakdown voltage may not be definitely established, thereby making it difficult to adjust the high voltage level. Thus, after the memory device is manufactured, it is impossible to adjust the high voltage level.